Composite structures have been known in the art for many years. Although composite structures can be formed in many different manners, one advantageous technique for forming composite structures is a fiber placement or automated collation process. According to conventional automated collation techniques, one or more ribbons of composite material (also known as composite strands or tows) are laid down on a substrate with a material placement machine. The substrate may be a tool or mandrel, but, more conventionally, is formed of one or more underlying layers of composite material that have been previously laid down and compacted.
Conventional fiber placement processes utilize a heat source to assist in compaction of the plies of composite material at a localized nip point. In particular, the ribbon or tow of composite material and the underlying substrate are heated at the nip point to increase the tack of the resin of the plies while being subjected to compressive forces to ensure adhesion to the substrate. To complete the part, additional strips of composite material can be applied in a side-by-side manner to form layers and can be subjected to localized heat and pressure during the consolidation process.
Unfortunately, inconsistencies can occur during the placement of the composite strips onto the underlying composite structure. Such inconsistencies can include tow gaps, overlaps, dropped tows, puckers (i.e., raised regions in a tow), and twists. In addition, there are foreign objects and debris (FOD), such as resin balls and fuzz balls, that can accumulate on a surface of the composite structure which must be detected, identified and eventually removed from the ply surface.
Composite structures fabricated by automated material placement methods typically have specific maximum allowable size requirements for each inconsistency, with these requirements being established by the production program. Production programs also typically set well-defined accept/reject criteria for maximum allowable number of (i.e., density) of inconsistencies-per-unit area and maximum allowable cumulative inconsistency width-per-unit area.
To ensure that the composite laminates fabricated by fiber placement processes satisfy the requirements pertaining to inconsistency size, the structures are typically subjected to a 100% ply-by-ply visual inspection. These inspections are traditionally performed manually during which time the fiber placement machine is stopped and the process of laying materials halted until the inspection and subsequent action to address the inconsistencies, if any, are completed. In the meantime, the fabrication process has been disadvantageously slowed by the manual inspection process and machine downtime associated therewith.
Recently, systems have been developed that are capable of detecting, measuring, and marking individual inconsistencies in the composite structure. Exemplary systems and methods capable of accurately and reliably detecting, measuring and/or marking inconsistencies in a composite structure are disclosed in U.S. patent application Ser. No. 09/819,922, filed Mar. 28, 2001, U.S. patent application Ser. No. 10/217,805, filed Aug. 13, 2002, and U.S. patent application Ser. No. 10/628,691, filed Jul. 28, 2003. The entire disclosures of U.S. patent application Ser. Nos. 09/819,922, 10/217,805, and 10/628,691 are each incorporated herein by reference as if fully set forth herein.
Although these inspection systems have worked well for their intended purposes, the inventors hereof have recognized that it would be even more beneficial to provide systems and methods that are capable of determining an inconsistency characteristic of a composite structure, such as the composite structure's inconsistency density-per-unit area and/or cumulative inconsistency width-per-unit area.